This invention relates to a system for protection against copying on magnetic tape recorders. The expression “tape recorder” here includes both audio and video tape recorders.
Music artistes and other owners of copyright works have been plagued by unauthorised copying of their work. For example much music and other audio programme material is now available recorded on compact discs (CDs) and is nevertheless readily copied by the use of a simple commercially-available analogue cassette tape recorder. The same is true for other recording formats as well as for broadcast programme material, both in audio and video form.
Attempts have been made to render recordings such as CDs uncopyable. Generally these have involved recording on the CD an additional tone which is outside the range of audible frequencies but which nevertheless interferes with the high frequency bias oscillator frequency of the tape recording head in the tape recorder, thereby creating distortion in the tape recording which is unpleasant to listen to when it is replayed. The additional tone may be a low-frequency tone below the audible range or may be a high frequency tone above the audible range.
For example, U.S. Pat. No. 4,086,634 describes a system in which a high-frequency signal outside the normal hearing range is frequency modulated onto the original signal to be recorded. When an attempt is made to copy the recorded material onto magnetic tape, detectable interference signals, such as beat signals and cross-modulation signals, are said to arise between the recorded modulation signal and the copying tape recorder's recording bias. These interference signals will arise at least in part within the audible range and will ruin the replay of the recorded tape. Nevertheless the original recording can be replayed without degradation. In this US patent, the superimposed signal is applied in portions of the original signal where the original signal has at least a predetermined magnitude. The superimposed signal preferably varies in a predetermined manner so as to sweep cyclically over a selected spectrum of frequencies.
U.S. Pat. No. 5,822,360 describes a system for transporting auxiliary data in a conventional audio signal. The auxiliary data is hidden in the audio signal in the form of coloured noise.
The data to be transported is first converted to a spread spectrum signal. The primary audio signal is analysed to determine its spectral shape. The same spectral shape is imparted to the spread spectrum signal, which is then combined with the primary audio signal for transmission.
Another system is described in U.S. Pat. No. 4,644,422 in which an interfering signal is added to the original recording, the interfering signal in one example being the output of a pulse width modulator which is combined with the source only when a selected parameter, notably the frequency, of the source signal, exceeds a specified threshold frequency.
Other proposals have been made in French Patent Application FR-A-2 740 897 and U.S. Pat. No. 5,523,853.
In practice it has been difficult to make such a system work effectively. The constraints on the system are substantial; the added signal has to destroy the recorded material when it is re-recorded on a tape recorder sufficiently to render it unplayable so far as the listener is concerned, while not causing any noticeable deterioration in the high-quality original recording when that is replayed. All this has to be achieved using conventional unadapted CD players and without any modification having to be made to commercially available tape recorders.
We have found that various problems arise with existing proposed systems. For example, the signal-to-noise ratio (S/N) can be deleteriously reduced, due to the addition of a continuous anti-copy signal. Some systems can have the effect of reducing the dynamic range of the audio signal due to the anti-copy signal. For certain kinds of signals, some systems have the results that either the original recordings do not play well, or the effectiveness of stopping re-recording is insufficient. The playability of the recording can be poor. Finally, some systems have the effect of increasing the power consumption of the player of the original recording causing heating in the power transistors and speakers.
While described in the context of CDs, similar problems arise with other programme material, such as video recordings or sound and/or vision broadcasts. In the field of audio, recordings can include music of various types, speech, song and periods of silence.